The invention relates to a brake control system and method for motor vehicles with an electronic control unit, by which, when the motor vehicle is stationary, a parking brake function can be activated manually or automatically. Deactivation of the parking brake function occurs upon reaching a predefined release condition.
A plethora of various patent applications or patent documents, such as DE 199 50 034 A1, DE 196 11 359 C1, DE 199 50 162 B4, DE 198 48 448 C2, DE 101 51 846 B4, DE 199 41 482 A1, etc., disclose methods and devices that are supposed to prevent vehicles from rolling away—in particular, but not necessarily, on slopes. Therefore, in at least one operating state upon actuating the brake pedal or by use of another operating control element, a braking force is usually held, or actively built-up, at least at one wheel of a vehicle as a function or independently of, the magnitude associated with operating the pedal or the other operating control element. In order to activate or deactivate the function, various conditions for fulfillment or initiation are provided as a common feature or as an alternative.
These methods or devices exist, for example, under the names start assistant, hill start assistant, hill hold(er) or automatic hold. In summary, these methods are referred to below as automatic parking brake functions. The common feature of all of these automatic parking brake functions is, for example, a hydraulic, mechanical, electro-hydraulic, electro-mechanical, or pneumatic brake system that can be controlled electronically and with which the vehicle can be held stationary or can be prevented from rolling away. There also exist methods that prevent the vehicle from rolling away by use of the transmission (in particular, an automatic transmission). In all of these functions, the activation of the function is performed, for example, by actuating the brake pedal or the hand brake or by means of the shift elements, which are connected to the brake pedal or hand brake, or any other operator control elements (for example, switches, push buttons, and the like). A deactivation is performed by means of the same or other auxiliary tools (such as the gas pedal position, coupling signal, drive torque signals of the engine, and the like, or also following passage of a defined holding period).
Other important necessary auxiliary tools, control signals or switching devices for carrying out the automatic parking brake functions are, for example, usually one speed signal (or a plurality of speed signals) for the detection of the vehicle standstill; mechanical, pneumatic or hydraulic actuators (in principle, electro-hydraulic, electro-pneumatic or electric actuators could also be used) in the wheel brakes or in the feed lines to the wheel brakes, by which the braking effect can be increased or decreased or at least held temporarily (for example, control/adjusting valves integrated in the form of an existing control or adjusting device, for example, ABS, ASC/ASR, DSC/ESP/EHB system); optionally, also longitudinal acceleration or tilt sensors, by which the longitudinal slope or grade of the road is determined and from that the necessary braking force for a secure standstill of the vehicle and the starting torque, which is necessary for the grade of the road, can be determined; and one pressure sensor (or a plurality of pressure sensors), by which the brake pressure or the generated brake torque at the wheel brake(s) can be determined.
The invention improves upon the transition from a parking brake function into a start operation.
According to the invention, a brake control system and method for a motor vehicle is provided. The brake control system has an electronic control unit, by which, when the motor vehicle is stationary, a parking brake function can be activated manually or automatically. Deactivation of the parking brake function occurs upon reaching a predefined release condition. In the presence of a release condition, the brake pressure, which was built-up for the parking brake function, is released in a time offset manner at least in relation to the axle by way of the control unit. The time offset manner could be, for example, a phase or amplitude offset. Advantageous further developments of the invention are described and claimed herein.
The present invention is based on the following additional knowledge. An uncomfortable behavior when starting a vehicle from a full stop can occur with the prior art methods and devices (in particular, in connection with the aforementioned brake systems) in the following situations:
When decelerating the vehicle on a downhill descent out of the drive mode as far as to a full stop of the vehicle, stress and strains are generated, for example, as a function of the construction of the vehicle, the chassis and/or the brake system. In principle, the greater the road ascent and, thus, the slope descending force and the greater the delay and the stopping jolt until reaching the standstill, then the higher these stresses are. In particular, it must be pointed out that the stresses at the axles of the vehicle can increase to varying degrees owing to the varying elasticity. With a change in the desired direction of travel that takes place after the vehicle has been at a standstill (e.g., detectable by way of a gear change) and when starting from the hill-hold function (for example, start assistant, automatic hold, ACC stop and go, park maneuvering assistant), the practice to date has been, according to the prior art, to release the brake torque at the wheel brakes simultaneously with an adequate drive torque (for example, brake pressure decreased at all four wheels), so that the vehicle can be put into motion.
As a result, stresses of varying magnitude are released at the vehicle axles; and partially negative acoustical stress relief noises and driving comfort-sided effects (release and/or starting jerks, no homogeneous and uniform release and starting behavior, “wind-up effect”) occur.
When driving forward downhill, as well as when braking in the standstill and subsequently changing the direction of travel (backwards uphill), the invention proposes, for example, that a start assistant or another system with a parking brake function no longer release the brake pressure simultaneously at the wheel brakes.
The procedure will also be the same when decelerating into the standstill, when the vehicle is standing backwards downhill and then there is an abrupt change in the direction of travel, and when the vehicle starts up from a full stop and travels forward uphill.
Therefore, the inventive method includes the following measures. When the brake torque is released axle-wise (slightly) time-offset (for example, phase offset or amplitude offset), in that the axle experiences (as a function of the construction of the vehicle, as is well known) the higher stresses in the respective driving situation (dependent on the direction of travel, that is, the vehicle is standing forwards or backwards downhill), the brake torque decrease is started at an early stage at the wheel brakes of the other axles. The premature release of the brake torque at an axle can take place suddenly or linearly (in a ramp-like fashion) or according to another functionality. Even a stepped release to one or more temporary intermediate holding torque levels (holding pressure levels) with a subsequently additional holding torque decrease is also contemplated.
The premature decrease in the brake torque at the wheel brakes of the one axle or its time advancement compared to the decrease in the other axle can be, for example, dependent one or more of several factors, such as: (a) dependent on the ascent of the road, (b) dependent on the brake delay, which occurs upon stopping, in the standstill, (c) dependent on the brake jerk, which occurs upon stopping, (d) dependent on the brake torque level (dependent on the brake holding pressure level), (e) dependent on the vehicle mass or load (dependent on the trailer), (f) dependent on the brake temperature, (g) dependent on the brake wear and tear, (h) dependent on the driving pedal, and/or the driving pedal gradient, (i) dependent on the drive torque and/or the drive torque gradient, and (j) dependent on the engine speed and/or the engine speed gradient. Other dependencies are contemplated as well, for example, dependent on the gear steps or dependent on the drive train gear ratio.
In the event of a detected so-called “μ-split lane” (ratios of the coefficient of friction that vary as a function of the side of the vehicle), it is contemplated that the brake torque decrease may vary for the selected wheel or for the selected side.
The invention achieves a significant, perceptible, and audible improvement in the starting comfort with respect to a parking brake function on slopes in the event of a preceding change in the direction of travel (avoidance/reduction in the acoustical impairments and an improvement and/or increase in the starting comfort, i.e., jolt-free starting from a full stop) due to a brake torque release that is time offset for the selected axle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.